This invention relates to control systems for d-c feeder breakers employed in transit type systems, and more specifically relates to a novel automatic reclosing system which has improved system impedance measurement sensitivity and improved operation over a wide range of system voltages.
Control systems for d-c feeder breakers in transit-type systems are well known. One such system is disclosed in U.S. Pat. No. 4,232,232 in the names of DeLacy and Lechner entitled "VOLTAGE SENSING AND LOAD MEASURING AUTOMATIC RECLOSING SYSTEM FOR D-C CIRCUIT BREAKERS". The system shown therein provides a method for measuring transit system conditions before permitting the closure of a d-c feeder breaker. Thus, the system first automatically measures rail voltage. Closing of the feeder breaker is then permitted if the feeder voltage is above, for example, 400 volts, since this measurement indicates the absence of a fault. If, however, the monitored voltage is lower than 200 volts, the integrity of the feeder system is further measured by injecting a current into the system and measuring the voltage drop across the feeder circuit produced by the injected current. If the voltage drop measured is greater than 200 volts, there is no fault on the feeder circuit and the closing of the feeder breaker is permitted. If the measured feeder voltage is intermediate the high and low voltages referred to above, which permitted feeder breaker closure, and initiated the load measuring mode, respectively, the breaker is prevented from closing. Moreover, if, during the impedance measurement carried out in the load measuring mode, it is found that the impedance of the feeder circuit is lower than a given value, which would indicate a short circuit in the feeder circuit, the breaker is prohibited from closing. The above measurement modes are repeated automatically a given number of times. If breaker closure does not occur after a given number of attempts, the breaker system is locked out.
While the above system works very well, the measurement process cannot be used on a feeder circuit in older transit systems when trains are parked on a deenergized section of the circuit. These trains form a relatively low impedance between the third rail and the tracks of such systems which might be approximately 0.2 ohm for an eight-car train.
The prior art circuit described above does not measure impedance low enough in the load measuring mode to differentiate the 0.2 ohm load from a faulted circuit. Thus, the existing circuit can operate improperly in systems in which there is a very low impedance load which might be interpreted as a short circuit by the measuring system.